1. Field of the Invention
The present invention relates in general to a microscope objective lens usable in usual bright field observation as well as in fluorescence observation and in particular to a microscope objective lens with a large numerical aperture having excellent performance in fluorescence observation by means of near-ultraviolet excitation (excitation wave-length: 340 to 380 nm).
2. Description of the Related Art
Recently, study has been much conducted into calcium ions having vital effects in organisms, for which purpose fluorescent microscopes have been used to measure time-wise concentration variations of calcium ions in the cell. In such measurement, an ultraviolet light with a near ultraviolet wave-length of 340 to 380 nm is applied as an excitation light to an observation target, so that fluorescence caused by the ultraviolet-light excitation may be observed. When, in such a way, fluorescence is observed by a typical application-type fluorescent microscope in which the objective lens acts also as a condenser, the excitation light is applied through the objective lens onto an observation target, to observe fluorescence generated by the target. Therefore, the objective lens in this case requires the following special arrangement with respect to near-ultraviolet light (excitation light).
First, the objective lens needs to provide a sufficient transmission factor (transmissivity) in the near-ultraviolet light range, i.e. the range of the excitation light. A typical conventional objective lens suffers from a rapid decrease in transmission factor against ultraviolet-range light, and it is difficult to excite a sample(observation target) with a near-ultraviolet light, so that an objective lens having a high transmission factor for near-ultraviolet light is desired. It is also necessary to inhibit as much as possible self-emission and solarization which are observed when an excitation light is applied to the glass material of the objective lens. Moreover, since the intensity of fluorescence caused by the application of an excitation light is extremely small as compared to the intensity of the excitation light, the objective lens needs to have a very high numerical aperture so as to be able to capture such a small intensity of fluorescence. Thus, a fluorescent objective lens is very difficult to design, because the types of glass materials that can be used are limited and a high numerical aperture is required.
This a type of fluorescent objective lens has already been proposed in a few disclosures, for example, laid-open patent publication Sho 55-79406 and laid-open patent publication Sho 55-79408. The objective lenses disclosed in those laid-open publications are made of glass taking into consideration the transmission factor in the near-ultraviolet light range and are designed to be used as the above-explained fluorescent objective lens.
However, the above-mentioned microscope systems corrects magnification chromatic aberration generated by the objective lens at the eye-piece, thus suffering from the disadvantage that these systems are effective only by combining the objective lens with a prescribed eye-piece. That is, in the present microscope systems, the objective lens and the eye-piece each correct magnification chromatic aberration independently of each other, so that such microscope systems using any objective lens disclosed in the above-mentioned laid-open publications suffer from the disadvantage that magnification chromatic aberration generated by the objective lens will remain uncorrected.
Both laid-open patent publications Hei 5-142477 and Hei 7-230039 (especially, embodiments 1 and 2) also disclose objective lenses suited for fluorescence observation. However, since the objective lenses disclosed in these publications are of a liquid immersion type, these objective lenses have a larger numerical aperture but are inferior in convenience to dry-system objective lenses. Moreover, in the objective lenses disclosed in these publications it is difficult to reduce the Petzval's sum, so that a complicated arrangement is required for the relevant lens system to achieve a certain desired degree of flatness of the image surface, which increases the cost of the system.
Also, embodiments 3 and 4 of laid-open patent publication Hei 7-230039 disclose dry-system objective lenses which have a large numerical aperture and are well corrected in terms of secondary dispersion. However, in these systems, if the image height exceeds 5.5 mm, the comatic aberration worsens, which is problematic for wide-range observation.
As noted above, in order to improve the transmission factor of excitation lights in the near-ultraviolet range (e.g., 340 nm), only a few glass materials having a high refractive index and a high dispersion can be employed. Therefore, in designing objective lenses with large numerical apertures made of limited types of glass materials, imaging performance has inevitably been sacrificed to some extent.
In consideration of these problems, it is an object of the present invention to provide a microscope objective lens with a large numerical aperture that is made of a glass material having a high transmission factor in the near-ultraviolet range and that is capable of properly correcting spherical aberration, comatic aberration, and chromatic aberration including secondary spectrum and also that provides imaging effective up to the view field periphery for the case of bright field observation as well as fluorescence observation.